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One of Saturn's moons looks a lot like an infamous planet-destroying battle station from science fiction, but astronomers have some very real theories about the complex crater that gives Mimas its unique feature.

Hosted by: Reid Reimers

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I'm gonna cut off all you Obi-Wan Kenobis before you start.  Yes, this totally is a moon.  It's the smallest major moon of Saturn, Mimas, and thanks to the laws of physics and a bit of luck, it looks freakishly like the Death Star.  This tiny icy moon would be unremarkable to everyone but the most dedicated astronomers if it didn't resemble an iconic weapon from a beloved Sci-Fi franchise.  But the story of how this very real moon came to look like an infamous not-moon-super-laser-equipped-battle-station is actually pretty cool.

The resemblance is totally coincidental.  British astronomer William Herschel discovered the moon in 1789 and named it after a figure from Greek mythology.  That giant crater is now called Herschel Crater in his honor, but it actually wasn't spotted until 1980, a few years after Star Wars hit theaters.  

Mimas looks greyish-white because a lot of things in space look greyish-white.  It's made mostly of water ice, which reflects sunlight.  Astronomers think it may get (?~1:03) of fresh ice crystals from Enceladus, another moon of Saturn that's known for its icy geysers.  The ice crystals dripped off into Saturn's outer rings and Mimas sweeps them up as it passes by.  Size-wise, Mimas is actually bigger than the Death Star, at least the first one.  The first Death Star had a diameter of around 160km.  Well, Mimas comes in at 396kms.  Since Mimas is Saturn's smallest moon, the first Death Star was actually kind of small by moon standards.  Apparently, the diameter of the second Death Star is less clear and it could have been as big as 900 km, so it might have made a decent sized moon.  

Herschel Crater, meanwhile, is 139 kms wide and no, I don't know how big the laser generator is, but someone's probably Tweeting it to us right now, ya nerds.  We don't know what hit the moon to create the crater, although astronomers figure it happened about 4.1 billion years ago.  139 kms is a lot of crater.  It's about a third the diameter of the moon itself.  In fact, Herschel Crater is probably as big as it could possibly be, proportionally speaking.  The impact that created the crater nearly blew Mimas to Alderaan sized bits.  You can see the stress fractures from the crash on the other side of the moon.  

There's also a stumble to Mimas' orbit known as a libration in astronomical terms.  That makes scientists think it's not totally uniform on the inside.  Some have speculated that the libration is caused by an ocean inside Mimas, but that's mostly been ruled out.  It's probably just a result of the impact.  Mimas got slammed into so hard that the impact rearranged its guts, left it reeling.  

Craters tend to have certain characteristics based on their size and Herschel Crater is what's formally known as a complex crater.  That means it's big enough to have a feature called a central peak.  The peak superficially resembles the dimple in the Death Star's super laser, but it's only there because of the physics of things smashing into each other.  When a meteor hits, it carves out a basin much bigger than the meteor itself.  Meanwhile, the rock right at the center of the impact rebounds and bounces back up.  Ever toss a rock into a pool of water and see that water bounce back up in the middle?  It's kind of like that.  The material that's pushed out toward the edges of the impact area will slump back in.  That's the technical term, and pile up in the center.  That's how craters like Herschel come to have central peaks.

Herschel Crater's peak is around 6kms high.  If that figure sounds familiar, that's because the peak is roughly the size of Mt. Everest.  That means there's an Everest-sized mountain on a moon whose diameter is about the same as the distance from New York to Washington DC or 2.5 Death Stars, ya nerds.

It's no wonder Herschel Crater completely dominates such an otherwise tiny and unremarkable moon.  Another thing that makes it different from the Death Star is that Mimas has never destroyed a planet, but it does have enough gravity to affect Saturn's ring system.  Its gravitational pull is what cleared the gap in the rings called the Cassini division, which is also the name of my new techno band.

So even though Darth Vader has never prowled the icy depths of Mimas, there's still some neat science just below the surface.  Also on the surface and six kms above it.

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